Owing to its small molecular size, its dielectric constant, its electron donor properties and its ability to form complexes, dimethylformamide is outstandingly suitable as a solvent, especially for high-molecular compounds. It is therefore one of the few solvents suitable for the preparation of spinnable polyacrylonitrile solutions, but the use of dimethylformamide as solvent is also indispensable in the synthetic leather industry and coating industry for the preparation of polyurethane systems for high-quality surfaces. Certain polyamides, especially fully aromatic polyamides, can be spun only in the form of their solutions in DMF or similar solvents.
Owing to its selective solvent power for certain hydrocarbons and gases, dimethylformamide is used in a number of large-scale industrial processes. Thus, for example, acetylene can advantageously be extracted by means of DMF even from cracking gases low in acetylene, and acetylene-free ethylene can be obtained simultaneously.
In the last few years, DMF has even been used as a selective absorbent for butadiene from the C.sub.4 fraction of cracking gases. In the petroleum industry, it is used as a selective extractant for aromatics from hydrocarbon mixtures, and for the desulfurization of diesel oils and lubricating oils, and the outstanding solvent and extraction properties of DMF for vitamins, hormones, sulfonamides and antibiotics are also exploited in the pharmaceutical industry.
DMF thus represents an irreplaceable auxiliary in a multiplicity of large-scale industrial processes.
In the event of faults in plant operation or also during certain operating conditions close to the limits of these processes, small or very small quantities of dimethylformamide or its hydrolysis product dimethylamine can pass into the effluent from these production processes and must then be removed from these for ecological reasons. However, this necessity presupposes unbroken monitoring, as far as possible, of the concentration of dimethylformamide and dimethylamine in the plant effluents. For this purpose, an analysis method, which can be handled as easily as possible and gives quick results, for the determination of dimethylformamide and dimethylamine in aqueous solutions is required.
In the past, the hydrolysis with acid has proven itself in particular as a specific method for the determination of dimethylformamide in aqueous solutions. The dimethylamine formed is then separated by distillation from an alkaline solution and nitrosated, and the resulting nitroso compound is determined polarographically. Although this analysis method is relatively accurate, it is too time-consuming and involved for continuous checking of plant effluents.
An example of a further commonly used method for detecting or also for checking the purity of dimethylformamide is gas chromatography. This method also presupposes frequent taking of individual samples and analysis of the latter, and is therefore not really suitable for continuous effluent monitoring. A further known method, which has a low detection limit, is easy to apply and gives rapid and accurate results, is based on the measurement of the electrical conductivity of aqueous solutions containing dimethylamine. However, even this measurement principle has hitherto been used only for the discontinuous determination of the dimethylamine concentration in aqueous solutions. The conventional sample preparation requires the presence of individual samples and presupposes that other substances which conduct electric current are absent.